The cristobalite phase of aluminum phosphate (AlPO4) ceramic exists in two modifications, the low temperature cristobalite (denoted as, low cristobalite, or α-form) and the high temperature cristobalite (high cristobalite, or β-form). The two modifications are separated by a reversible phase transformation that occurs at about 270° C. The phase transformation results in abrupt volume and structural changes and discontinuous thermal expansion, which are not conducive to technological applications. Structures of the α- and β-phases have been reported by various researchers including Wright and Leadbetter (Phil. Mag. 31, 1391, 1975). AlPO4 is isomorphous with silica and exists with silica in various forms including the α-cristobalite form, with phase transformations at similar temperatures. The structure of the alpha form of AlPO4 is tetragonal, a=b=5.03 Angstroms and c=7.00 Angstroms with space group C2221. The structure of the α-AlPO4 phase is similar to corresponding silica structures with Al and P atoms alternately replacing the silicon atoms. (Mooney, Acta Cryst. 9, 728, 1956) The structure of β-AlPO4 shows a cubic structure, space group F-43m, with a ˜7.2 Angstroms.
It is well known in the glass ceramics field that high temperature forms of silica can be formed at relatively low temperatures by stabilizing the ceramic matrix with dopants. The silica counterpart of the aluminum phosphate materials described above (tetragonal α-cristobalite) undergoes a displacement phase transition to the cubic high temperature β-phase at about 300° C. Various reports regarding the stabilization of cristobalite phases of silica ceramics by various methods have been issued. U.S. Pat. No. 5,096,857, M. A. Saltzberg, et al., J. Amer. Ceram. Soc. 1992, 75, 89, and P. L. Gai, et al., J. Solid State Chemistry, 1993, 106, 35, all describe chemically stabilized solution-derived silica β-cristobalite of the type CaxAlySi1-x-yO2 and its compositions. R. D. Shannon, et al., Phys. and Chem. Miner. 1992, 19, 157, reported compositions in the BPO4/AlPO4/SiO2 system (BAPOS), with compositions up to 75% AlPO4, 75% SiO2 and 50–60% BPO4. A relatively high amount (approximately 15%) BPO4, was used in these studies. The authors reported the presence of secondary amorphous phases (i.e., the materials were not single phase), and suggested that stabilization could be achieved using only framework ions (i.e., no ions in the interstices).
M. Rokita, et al., Pr. Kom. Nauk. Ceram. Pol. Akad. Nauk 1997, 54, 161 describe the synthesis of solid-solutions of SiO2—AlPO4. A single dopant, 20–75% mole % SiO2, was used. The structures and compositions of the solid solution with this single dopant (SiO2) were not determined because the solid solution formed multiphasic systems. Also, a relatively large mole percent (20–75 mole %) of the dopant SiO2 is used in this work. M. Handke, et al., Vib. Spectr, 1999, 19(2) 419–423 show spectroscopic data from these compounds and demonstrate that multiphasic systems are formed.
Stable ceramic materials are required for a number of end-uses, including use as piezoelectrical materials (i.e., structured materials which produce electric polarization when mechanical stress is applied), as stable supports in catalysis and biotechnology, as ceramic fillers with low dielectric constants in electronic application and as ceramic coatings for reactor materials.
In view of the foregoing, it is advantageous to develop a stable ceramic material that is single phasic through a wide range of temperatures.